Although conventional dielectrics used in manufacturing printed circuit boards, such as polytetrafluoroethylene (PTFE), are capable of tolerating high temperatures generated by high power and high wattage components, the generated heat must be dissipated in order to prevent the high temperatures from affecting the operation of the circuitry and possibly damaging the components or surrounding circuitry.
Initial attempts at solving this problem have resulted in designing pre-bonded metal-backed PTFE printed circuit boards. Metal-backed PTFE circuit boards began as a single-sided printed circuit board having a cladding or foil plating of conductive material, such as copper, on a front side of a dielectric and a metal backing or support on a backside of the dielectric. The metal backing was simply affixed to the backside of the dielectric material using a non-electrically conductive adhesive. The foil plating was patterned and etched to form printed wiring for securing electronic components, and the metal backing functioned as a heat sink.
Later attempts became more sophisticated and circuit boards were designed that incorporated electrically conductive or plated through holes (PTHs) that passed completely through the printed wire foil, the dielectric, and the metal backing, thus enabling the metal backing to function as a ground plane as well as a heat sink. Copper was the first metal used as a heat sink because of its compatibility with the plated through hole process. Aluminum was later used because of its ease in machining, lighter weight and lower cost. Aluminum now accounts for the majority of metal-backed PTFE circuit board designs, but the plating of aluminum backed PTFE circuit boards having plated through holes is generally a relatively expensive process.
A notorious problem associated with the pre-bonded metal-backed circuit board design is that the entire board must be processed as one assembly. Since the pre-bonded circuit board has plated through holes that extend completely through the thicknesses of the foil plating, the dielectric and the metal backing, these materials must first be bonded together before drilling holes in order to ensure that the plated through holes are correctly aligned. This construction method of the pre-bonded circuit board significantly limits the design options available to a design engineer because the dielectric and the printed wiring of the circuit board cannot be constructed independently from the metal backing. The metal backing must be permanently affixed to the dielectric during the initial construction of the circuit board, and thus be carried by the dielectric during the remainder of the circuit board construction procedure.
An alternative approach used to avoid the problems associated with pre-bonded circuit boards has been to build the circuit board separately from the metal backing as a double-sided board and then "sweat" the two together using solder paste. This attempted method, however, is very messy and the resulting bond is unreliable and tends to delaminate if subjected to vapor phase soldering and assembly.
A more recent attempt at designing a circuit board with a heat sink ground plane produces a double-sided PTFE circuit board having plated through holes extending through front and back foil platings on opposing sides of a dielectric. The back foil plating is coated in accordance with conventional procedure, and is configured to function as a ground plane. The coated back foil plating is then conductively bonded to a metal support or backing with a conductive adhesive. The conductive adhesive carries current to the metal support, or heat sink, in an attempt to eliminate the need for plated through holes in the heat sink.
While this design, at least in theory, appeared to provide a better alternative to some of the problems associated with prior attempts at designing a metal-backed circuit board, practical application of this design proved to create new problems. For example, in order to ensure the conductive quality of the foil plating used to form printed wiring, it is standard practice for all foil platings to be coated with a layer of protective material during production and prior to shipping. The coating is necessary in order to prevent a conductive foil plating, such as copper, from oxidizing or developing a film coating of impurities, thus becoming a less effective conductor than pure copper. The protective coating also functions to improve solderability of electronic components to the foil plating. The protective coating is typically a form of tin-lead that will melt when electronic components are soldered to the printed wiring of the circuit board.
As a result of the required protective coatings applied to the foil platings of circuit boards, the protective layer is positioned between the back foil plating and the conductive adhesive during the conductive bonding procedure. The presence of the protective coating being in contact with the conductive adhesive has negative effects on the quality of the finished circuit board. For example, the protective coating can melt, as it was designed to do, when exposed to heat, thus weakening or destroying the bond between the back foil plating and the metal support. Unfortunately, this exposure to heat occurs when the electronic components are being soldered to the front foil plating after the bonding procedure has been carried out. Furthermore, the protective coating can decrease the conductivity of the conductive bond by positioning a layer of less conductive material between the back foil plating and the metal support.
Commercially available plated dielectrics used in the production of circuit boards require the protective coatings in order to ensure the conductive quality of the platings, and the protective coatings cannot simply be removed without risking oxidation or the development of a film of impurities on the foil platings.